When a CMOS is fabricated, for example, an impurity region having a conductivity type that is different from or the same as that of a semiconductor substrate is formed in a surface of the semiconductor substrate, in other words, a so-called well structure is used. For this well structure, an impurity concentration of the well is desired to be increased to a relatively high level in view of optimizing the channel length or preventing latch-up. On the other hand, when a high breakdown-voltage transistor is formed in the well, an impurity concentration of the well is desired to be lowered in order to secure high drain breakdown voltage and well breakdown voltage. In this way, when a high breakdown voltage transistor is formed in a well, mutually contradicting factors are required with respect to the impurity concentrations of the well.
It is an object of the present invention to provide a semiconductor device that can secure a high breakdown voltage and yet optimize its wells, and a method for manufacturing the same.